Illumination systems may be found in many different applications, including image projection display systems, backlights for liquid crystal displays and the like. Projection systems usually use a source of light, illumination optics to pass the light to one or more image-forming devices, projection optics to project the image(s) from the image-forming device(s) and a projection screen on which the image is displayed. The image-forming device(s) are controlled by an electronically conditioned and processed video signal.
White light sources, such as high pressure mercury lamps, have been, and still are, the predominant light sources used in projection display systems. In a three-panel image-projection system, the white light beam is split into three primary color channels, red, green and blue, and is directed to respective image-forming device panels that produce the image for each color. The resulting primary-colored image beams are combined into a full color image beam that is projected for display. Some other projection systems use a single imager panel, and so rotating color wheels, or some other type of time-sequential color filter, is used to filter the white light so that light at one primary color is incident on the image-display device at any one time. The light incident at the panel changes color sequentially to form colored images synchronously with the incident light. The viewer's eye integrates the sequentially colored images to perceive a full color image.
More recently, light emitting diodes (LEDs) have been considered as an alternative to white light sources. In some cases, different illumination channels are powered by respectively colored LEDs, or arrays of LEDs. For example, blue LEDs are used to illuminate the blue channel and red LEDs are used to illuminate the red channel. Some types of image display device, such as a liquid crystal display (LCD), employ polarized light, whereas the LEDs produce unpolarized light, and so only half of the generated light is usable by the LCD. Furthermore, LEDs that operate in the green region of the visible spectrum are known to be relatively inefficient, compared to blue and red LEDs, and so many systems require more green LEDs than blue or red LEDs. This problem of inefficiency in the green portion of the spectrum is compounded when the light is required to be polarized.